DE3400932A1 - METHOD FOR PRODUCING SOLID CATHODES - Google Patents

Description

Process jzSur manufacture of solid-state cathodes

The invention relates to a method for the production of solid-state cathodes for a melt-flow electrolysis cell for the production of aluminum, at least the work surfaces being wettable by the deposited metal are.

For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which for the most part consists of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon base of the cell, with the surface of the liquid aluminum or a solid body that can be wetted by this forming the cathode. Anodes, which are attached to the anode bar and are made of amorphous carbon in conventional processes, are immersed in the electrolyte from above. At the anodes, the electrolytic decomposition of aluminum oxide creates oxygen, which in carbon anodes combines to form CO2 and CO. The electrolysis generally takes place in a temperature range between 940 and 970 ^° C.

It is known that with large currents the interaction from vertical components of the magnetic field with horizontal components of the current to undesirable deformations the surface of the metal bath, which is a few centimeters high, and lead to undesirably strong metal currents can. With small interpolar distances, these undesirable deformations can become so large that the aluminum touches the anodes and causes short circuits.

The generated metal flow continues on the surface to an increased chemical dissolution or too fine dispersion of the aluminum in the melt flow, which is because of Reoxidation is known to result in a reduced current yield.

A lower current density, which in principle has an advantageous effect, would increase the cost of capital to an unacceptable extent for electrolysis cells and hall.

For some time, cathodes that can be wetted from liquid aluminum and have a significantly shorter interpolar distance have been known allow. The deposited aluminum forms a continuously draining one on these solid-state cathodes Movie. According to a further development, several solid-state cathodes arranged at a distance protrude per anode layout with a relatively small work surface out of the aluminum sump. The solid-state cathodes used exist mostly made of titanium diboride, a very expensive material. This is why it has been used in an industrial setting so far can not enforce.

The high costs of cathodes made from pure titanium diboride arise from the following three process steps:
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- Titanium diboride is produced at high temperatures by carbothermal or plasma processes

- Fine grinding is required for powder preparation

- Shaping and sintering effect due to the necessary Green body production as well as a difficult furnace management during sintering a high reject rate; energy and capital costs are high.

In principle, the high material costs for titanium diboride can be reduced in two ways:

- Titanium diboride is applied as a coating to a cheaper substrate, and / or

- the titanium diboride is made with a cheaper material at most thinned in such a way that the cathode as a whole is still wettable and has good electrical conductivity.

In DE-OS 2 305 281 a cathode or a cathode element for the fused-salt electrolysis for the production of Aluminum described, which on a surface a layer of a binary system of refractory material and a has a low proportion of carbon, which system was created from a eutectic of refractory hard metal and carbon is. However, among other things, the very expensive titanium diboride is used as the starting material, which makes it economical Use of cathode elements sets limits.

In U.S. Patents 3,661,736 and 4,308,114, a solid state cathode is used for the fused salt electrolysis of aluminum, which consists of a composite material. Refractory Grains made of a material that can be wetted by aluminum are embedded in a carbon matrix. To manufacture the Composite material is according to the first-mentioned patent

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fine carbon powder mixed with granular titanium diboride and treated with a suitable thermal process, according to the second-mentioned patent specification, granular titanium diboride mixed in tar and pitch. Such composite cathodes have a much lower durability than pure titanium diboride cathodes, because the carbon matrix comes into contact with the melt flow.

Furthermore, WO 82/01 018 describes a cathode made from a titanium diboride / graphite composite material described, which always has a high proportion of coal. Due to the in this publication The person skilled in the art can conclude that this is a titanium diboride / carbon eutectic either not contained at all or possibly as a dispersion within a carbon matrix is located. Obviously, a subsequent impregnation of the porous composite body is necessary in order to achieve a sufficient to achieve mechanical strength.

The inventors set themselves the task of developing a process for the production of wettable solid-state cathodes for To create fused-salt electrolysis cells for the production of aluminum, which is too essential while maintaining good quality cheaper products.

The object is achieved according to the invention in that titanium and boron-containing powdery starting materials with powdery Carbon are intimately mixed, the sum of this and that possibly withdrawn from a substrate Carbon in relation to the titanium diboride to be formed from the titanium and boron-containing starting materials

is metric, the mixture under a neutral or reducing atmosphere heated to a temperature between 1600 and 22Ö0 ° C temperature and left at this temperature for 5 to 45 then the run up of pores and channels reaction product at 2250 rpm, to 2600 ⁰ C. and while Leave at this temperature for 10 to 60 min to form a compact titanium diboride / carbon eutectic and then cool to room temperature.

The titanium diboride / carbon eutectic is during the second reaction step, heating to 2250 ~ 2600 ⁰ C,. a molten phase. This eutectic can be produced both as a layer on a substrate and as a shaped body.

With the aid of the two-stage process according to the invention compared to the standard production of titanium diboride the following savings can be made: 20

- The entire powder preparation, consisting of the chemical adjustment (high temperature step) and Fine grinding is no longer necessary.

- The melted mass only has to be sawn to size after sintering, thus eliminating the time-consuming process Shaping the green body. In the case of layers, there is also no need to saw, since the substrates are already the desired Have shape.

- The otherwise necessary high-temperature steps (carbothermia and sintering) are carried out by a single high-temperature process replaced, in which the conversion of the starting materials to T1B2 and the subsequent melt formation without intermediate cooling, takes place in the same device.

The first process step, also called carbothermal energy, preferably takes place at a reaction temperature between 2000 and 2200 "C. The substance formed in the subsequent second process step, the melt formation or formation of the titanium diboride / carbon eutectic, consists of intimately mixed titanium diboride and carbon components , has a dark metallic luster and contains approx. 8% by weight of carbon. Small amounts of TiC and B ₄ C can also be present in the eutectic structure.

Arc or induction furnaces in which both process steps are suitable are particularly suitable for carrying out the process can be carried out without intermediate cooling. Depending on the type of furnace used, the melted eutectic can also be poured in a known manner, e.g. by tapping or tilting.

The first process step is related to quality of the cathodes produced and their production costs are preferably carried out with the following starting materials:

Ή (> 2 + B2O3 + 5C ^ T1B2 + 5CO (1)

2TiO2 + B4C + 3C> - 2T1B2 + 4CO (2)

TiC + B2O3 + 2C 5 * T1B2 + 3CO (3)

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The titanium or boron-containing reaction components are in each of the equations in a stoichiometric relationship intimately mixed together. The finely powdered carbon, on the other hand, is added in a superstoichiometric ratio, preferably 5 to 20% by weight above. In all three chemical reactions, the emphasis is on the right-hand side, i.e. there is practically no reverse reaction.

In terms of economy, the reaction (1) is am cheapest, the starting substances titanium oxide and boron oxide are reduced directly. The use of boron carbide (2) instead boron oxide leads to an increase in costs, which is even more pronounced when using titanium carbide (3).

The two-step process for making a titanium diboride / carbon eutectic is, however, much more economical in all three of the first process stages listed above, than when commercial titanium diboride is mixed with carbon and thus in a manner known per se corresponding eutectic is produced.

Mechanically stable 1000 ^E C, and good electrically conductive material used as the substrate, for example, graphite of types AXF and ACF of the US company "Poco Graphite" or easily graphitized carbon .. It - in the manufacture of coated cathodes in electrolysis temperature of 900 is an expediently 0.5-5 mm thick layer of the intimately mixed starting materials is applied to at least one substrate surface.

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These mixed starting materials can be dry or can be applied as a slip. During the first stage of the reaction, the formation of the porous and permeated by channels Titanium diboride, there is no anchoring of the layer on the carbon substrate. In the second stage of the reaction however, the formation of the titanium diboride / carbon eutectic a liquid boundary layer forms, after cooling an anchoring occurs between the substrate and the now a compact titanium diboride / carbon eutectic.

When coating a carbon substrate with the mixture of the starting materials titanium oxide, boron oxide and carbon, less carbon is expediently added than in Case of the shaped body, because in the second reaction stage, the eutectic formation, the substrate depending on the reaction temperature and reaction time, more or less carbon is removed.

During the formation of the eutectic, the thickness of the coating is reduced to about half, pores and channels are largely eliminated: A compact layer is created that can be easily wetted by the liquid aluminum. Although this layer can be quite massive with a thickness of up to 3 mm, it must be regarded as extremely inexpensive.
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If from the intimately mixed starting materials according to Above reaction equations (1) to (3) moldings are to be produced, the powder mixture is in one Arc, induction or resistance for the time being in the first stage of the process reacted and then melted in the second reaction stage. The reaction

mass can preferably be allowed to solidify slowly and then produce the desired cathode shape with the appropriate release agents. The arc or induction furnace however, it can also be tiltable or partable, then shaped bodies are cast after the eutectic formation, which is preferred slowly 'cooled down. These moldings are essential cheaper to manufacture than when using direct eutectic formation from titanium diboride, e.g. as Granules with a grain size of 0.1 - 3 mm and carbon.

Tests have shown that both coated and solid cathodes with the titanium diboride / carbon eutectic are wetted by aluminum and that these are suitable for its production by means of fused-salt electrolysis. Due to the presence of around 8% by weight free carbon in the cathode or the working surface of the cathode the person skilled in the art found this effect with surprise.

Claims (10)

-XT- PATENT CLAIMS . / Process for the production of solid-state cathodes for a fused-salt electrolysis cell for the production of aluminum, whereby at least the working surfaces can be wetted by the deposited metal,
5 characterized in that Titanium and boron-containing, powdered starting materials are intimately mixed with powdered carbon, the sum of this and the carbon removed from a substrate, if any, with respect to the titanium diboride to be formed from the titanium and boron-containing starting materials, the mixture under a neutral or reducing atmosphere heated to a temperature between 1600 and 2200 ^e C and left at this temperature for 5-45 min, then the reaction product permeated by pores and channels is ^{heated to 2250 - 2600 e} C and for 10 - 60 min to form a compact titanium diboride / Leave the carbon eutectic at this temperature and then cool it to room temperature. 2. The method according to claim 1, characterized in that the first method step at 2000-2200 ⁰ C is carried out. 3. The method according to claim 1 or 2, cjadurctv characterized, that both procedural steps without an intervening Cooling can be carried out in the same arc, induction or resistance furnace. - YT- 4. The method according to at least one of claims 1-3, characterized in that the finely powdered starting materials T1O2 and B2O3 in a stoichiometric ratio and also C can be used in an amount in excess of this stoichiometric ratio by up to 20% by weight. 5. The method according to at least one of claims 1-3, characterized in that the finely powdered starting materials T1O2 and B4C in stoichiometric ratio as well C can be used in or up to 20% by weight above this stoichiometric ratio. 6. The method according to at least one of claims 1-3, characterized in that as finely powdered starting materials TiC and B2O3 in the stoichiometric ratio and C in or up to 20% by weight above this stoichiometric ratio Ratio lying amount can be used. 7. The method according to at least one of claims 1-6, characterized in that the intimately mixed raw materials are applied as a 0.5-5 mm thick, dry layer a carbon substrate can be given. 8. The method according to at least one of claims 1-6, characterized in that the intimately mixed starting materials as a 0.5 - 5 ram thick layer of slip on a carbon substrate. 9. The method according to at least one of claims 1 - 6, characterized in that the intimately mixed starting materials are placed in a tapping or tilting furnace for reaction brought, melted and then cast as a shaped body or allowed to solidify as a cast block. 10. The method according to at least one of claims 1-9, characterized in that the eutectic formed is slowly cooled.